Heat-sensitive recording materials are well-known, which utilize the color forming reaction between a leuco dye and a developer to produce recorded images by heat. Such heat-sensitive recording materials are relatively inexpensive, and the recording apparatuses therefor are compact and easily maintained. For these reasons, heat-sensitive recording materials have found a wide range of uses: they are used not only as recording media for the output of facsimiles and a variety of computers, printers of scientific measuring equipment, etc., but also as recording media for a variety of printers of POS labels, ATMs, CAD, handy terminals, paper for various tickets, etc.
When, however, such heat-sensitive recording materials come into contact with oil, and plasticizers such as films or the like, alcohol, water, etc., problems occur such as fading of the color of recorded images, background coloration, adhesion of residue to the thermal-recording head during recording and the like.
In order to avoid such problems, a heat-sensitive recording layer has previously had thereon a protective layer composed of, e.g., a water-soluble resin such as polyvinyl alcohol, starch, acrylic resin or the like and a pigment such as kaolin, calcium carbonate, amorphous silica, colloidal silica or the like (see Patent Documents 1 to 7). Pigments such as calcium carbonate and amorphous silica have particularly been used for preventing the adhesion of residue to the thermal head. For example, a heat-sensitive recording material comprising a protective layer principally composed of a resin and a filler with a Mohs hardness of 2.0 or less has been proposed which does not cause thermal-head wear and has less adhesion of residue to the thermal head (see Patent Document 1).
In recent years, heat-sensitive recording materials have frequently been used as printed ticket forms and like printed matter. Printing with ultraviolet curable ink, in particular, has been widely used, since it offers advantages as follows:
(1) Solvent-free, therefore safety guaranteed
(2) Fast drying speed due to ultraviolet drying
(3) Energy savings achieved by compact UV irradiators
(4) Lowered drying temperature that leads to less background fogging by heat, especially in heat-sensitive recording materials
However, protective layers with satisfactory properties have yet to be attained, because conventional protective layers present problems as follows:
(a) Low adhesion of ink to heat-sensitive recording materials causes printed surfaces to be easily removed by, for example, cellophane tape.
(b) During recording with a thermal head, ink fuses by heat and adheres to the thermal head, easily causing a sticking phenomenon.
(c) The thickness of an ink layer printed on the surface of the protective layer of a heat-sensitive recording layer attenuates the recording energy from the thermal head, resulting in lowered recording sensitivity.
Moreover, heat-sensitive recording layers are used in places that require quietness, such as medical institutions, libraries and the like. In such places, the generation of a loud noise during printing (i.e., noise produced from sticking) is problematic, so that heat-sensitive recording materials substantially free from sticking are demanded. Furthermore, in the medical institutions where alcohols and medical creams are used, if heat-sensitive recording layers are touched by hands with such chemicals, background fogging occurs. In order to prevent such background fogging, heat-sensitive recording materials are demanded that have excellent barrier properties against chemicals such as alcohols, medical creams, etc., as well as barrier properties against plasticizers contained in medical files for storing the heat-sensitive recording materials.
Sticking is a phenomenon caused when material in close contact with the thermal head fuses or softens via recording energy, and attaches to the head. This sticking phenomenon causes problems such as the generation of noise during paper feed, skipping of recording (i.e., some portions are left unrecorded), and the like.
If materials with high heat resistance are used for forming protective layers to solve these problems, problems will occur such as the deterioration of film formation and lowered barrier properties against chemicals and plasticizers contained in medical files.
Moreover, if porous pigments such as calcium carbonate, silica and the like are used in protective layers in order to reduce sticking by absorbing the material fused or softened via recording energy, the anti-sticking properties will be improved, whereas the barrier properties will become poor. The use of porous pigments also significantly reduces the sensitivity when applied in large amounts to improve the barrier properties. Accordingly, it has been very difficult to obtain a high balance of anti-sticking properties, barrier properties, and recording sensitivity.
Furthermore, the use of acetoacetyl-modified polyvinyl alcohol in protective layers has been proposed in many literatures (see Patent Documents 8 to 12). However, it has been difficult to obtain a high balance of anti-sticking properties, barrier properties, and recording sensitivity.    Patent Document 1: Japanese Unexamined Patent Publication No. 1993-147354    Patent Document 2: Japanese Unexamined Patent Publication No. 1995-9762    Patent Document 3: Japanese Unexamined Patent Publication No. 2000-118138    Patent Document 4: Japanese Unexamined Patent Publication No. 2000-238432    Patent Document 5: Japanese Unexamined Patent Publication No. 2002-240430    Patent Document 6: Japanese Unexamined Patent Publication No. 2004-223994    Patent Document 7: Japanese Unexamined Patent Publication No. 2003-191647    Patent Document 8: Japanese Unexamined Patent Publication No. 1984-106995 (claim 1)    Patent Document 9: Japanese Unexamined Patent Publication No. 1995-232477 (claim 1)    Patent Document 10: Japanese Unexamined Patent Publication No. 1996-230323 (claim 2)    Patent Document 11: Japanese Unexamined Patent Publication No. 2004-284029 (claim 2)    Patent Document 12: Japanese Unexamined Patent Publication No. 2004-358762 (claim 3)